The present invention relates to a method for measuring phase currents in an inverter, where controlled semiconductor switches invert direct current by pulse width modulation from an intermediate circuit to a three-phase alternating voltage, and where the phase currents are measured by measuring the intermediate circuit current when an active switch state creates an unambiguous relation between the intermediate circuit current and a phase current.
The measuring principle is known from EP 0 502 226 A1, among others. That document describes a detection circuit, which by detecting the phase voltages selects the periods of time when said unambiguous relation between the intermediate circuit current and a phase current exists. The detection circuit controls an integration circuit of the track-and-hold type with three channels, which are connected alternately to a sensor for the intermediate circuit current. One of the channels is controlled to integrate the contributions of the individual phase currents to the intermediate circuit current when there is a potential applied on the inverter output, that is that a voltage is applied that is different from zero. The resulting signal represents the effective current in a connected three-phase a.c. machine. The two other channels are controlled so that the integrated signals periodically follow each its own phase current, and these signals are combined with data on the switch state into an individual reconstruction of the three phase currents.
The known circuit and the measuring method realised in it involve quite extensive analog signal processing. Due to the principle of integration applied, the circuit has a low upper cut-off frequency, and therefore it will be less suitable for inverters with high dynamics, as for example for servo control applications.
Seen in relation to this, it is the object of the present invention to provide a method for measuring phase currents in an inverter which allows a higher upper cut-off frequency, and which can be realised with less circuitry complications than known before.